1. Field of the Invention
The present invention relates generally to air bridge printed circuit boards, and more particularly to bond pads for fine-pitch applications on air bridge circuit boards.
2. Disclosure Information
U.S. Pat. Nos. 4,404,059 to Livshits et al. (hereinafter "Livshits"), 3,801,388 to Akiyama et al. (hereinafter "Akiyama"), and 5,738,797 to Belke et al. (hereinafter "Belke"), all three of which are incorporated herein by reference, disclose methods for making printed circuit boards (PCBs) comprising a substrate with etched tri-metal-layer (ETM) circuitry thereon, wherein the circuitry includes a plurality of air bridge crossover circuits. Because such circuits are produced by various chemical etching steps, care must be taken not to make certain portions of the circuits too small or narrow; otherwise, these small/narrow portions may become delaminated from the rest of the circuit, either during etching (because of the aggressive undercutting of the etching process) or during processing/use of the PCB (because of thermal, mechanical, or other stresses encountered).
This presents a particular problem for fine-pitch ETM applications, where the bond pad portions of the circuit must be closely spaced, while also providing sufficient spacing between adjacent pads for circuit traces to pass therebetween. FIGS. 1-2 illustrate top and side section views of an ETM circuit for a non-fine-pitch application with a relatively wide pitch P1, FIGS. 3-4 illustrate a much tighter (but not quite fine-pitch) application where the centerline-to-centerline pad pitch P2 is smaller than P1 but with the pads having the same width, and FIGS. 5-6 illustrate an attempt at a fine-pitch application where the pitch P3 is smaller than P2, as well as the bond pad width being made smaller. Note that in order to get the pads of FIGS. 5-6 tightly spaced as desired, not only does the centerline-to-centerline pitch P3 need to be made small, but the width of the pads must be shrunk as well. Moreover, adding in the traces between adjacent pads requires the pads to be made smaller still. However, when the width of the pads gets below a certain size--say 35 mils for 2-mil/6-mil/2-mil tri-layer-metal circuits--there is a tendency for the top layer of each pad to delaminate from the middle layer, causing shorts, opens, and/or other circuit problems. It would be desirable, therefore, to provide a way to accommodate fine-pitch applications for ETM circuits which avoids this tendency toward top layer delamination.